#include<bits/stdc++.h>
#define ll long long
#define mid ((l+r)>>1)
using namespace std;
namespace IO{
    char buff[1<<21],*p1=buff,*p2=buff;
    char getch(){
        return p1==p2&&(p2=((p1=buff)+fread(buff,1,1<<21,stdin)),p1==p2)?EOF:*p1++;
    }
    template<typename T>
    void read(T &x){
        char ch=getch();int fl=1;x=0;
        while(ch>'9'||ch<'0'){if(ch=='-')fl=-1;ch=getch();}
        while(ch<='9'&&ch>='0'){x=x*10+ch-48;ch=getch();}
        x*=fl;
    }
    template<typename T,typename ...Args>
    void read(T &x,Args& ...args){
        read(x);read(args...);
    }
    char obuf[1<<21],*p3=obuf;
    void putch(char ch){
        if(p3-obuf<(1<<21))*p3++=ch;
        else fwrite(obuf,p3-obuf,1,stdout),p3=obuf,*p3++=ch;
    }
    void pc(const char *s){
    	while(*s)putch(*s),s++;
    }
    char ch[100];
    template<typename T>
    void write(T x){
        if(!x)return putch('0');
        if(x<0)putch('-'),x*=-1;
        int top=0;
        while(x)ch[++top]=x%10+48,x/=10;
        while(top)putch(ch[top]),top--;
    }
    template<typename T,typename ...Args>
    void write(T x,Args ...args){
        write(x),putch(' '),write(args...);
    }
    void flush(){fwrite(obuf,p3-obuf,1,stdout);}
}
using namespace IO;
const int N=1e6+5;
int n,cnt;
vector<pair<int,int>>s;
priority_queue<int,vector<int>,greater<int>>q,dq;
bool vis[N];int last[N];
void add(int x,int id){
	if(!vis[id]){
		cnt++;
		vis[id]=1;
		last[id]=x;
		q.push(x);
	}else{
		dq.push(last[id]);
		last[id]=x;
		q.push(x);
	}
}
signed main(){
	freopen("contest.in","r",stdin);
	freopen("contest.out","w",stdout);
	read(n);
	for(int i=1;i<=n;i++){
		int len;
		read(len);
		for(int j=0;j<len;j++){
			int x;read(x);
			s.push_back({x,i});
		}
	}
	sort(s.begin(),s.end());
	int ans=2e9;
	for(int i=0;i<s.size();i++){
		add(s[i].first,s[i].second);
		if(cnt==n){
			while(!q.empty()&&!dq.empty()&&q.top()==dq.top())q.pop(),dq.pop();
			ans=min(ans,s[i].first-q.top());
		}
	}
	write(ans);
	return flush(),0;
}

